This is a trick most of you probably don't know, and that I just learned the other day. I almost discovered it on my own while musing about an equation to figure out the digits of a multiple of 9.If you want to know if a number is evenly divisible by three, just add up all the digits. If the result is evenly divisible by three, then so is the originally number.

For example, let's take 1872, for example. Couldn't have been more random. Now add the digits (1+8=9, 9+7=16, 16+2=18). Eighteen is divisible by three, so 1872 is too.Amazing, right?EDIT: Didn't realize I wrote, "for example" twice until now.